1. Field of the Invention
The present invention relates to Stimulated Brillouin Scattering (SBS). More particularly, the present invention relates to using the limited bandwidth of SBS to selectively attenuate certain modulation sidebands of a RF modulated lightwave waveform, while not attenuating other modulation sidebands.
2. Background of the Invention
Many RF receivers such as those used for communication and radar systems are located in complex and densely populated electromagnetic (EM) environments. The signal emitted from one antenna can interfere with the reception of another antenna. For example, commercial receivers located near high-power transmitters such as television or radio stations are subject to substantial interference. The intensity of the interfering signals can be many orders of magnitude higher than the desired signals to be sensed by the receiving system, such as a radar system. The high power in the interfering signal components can saturate the amplifiers in the receiver and thus distort desired signals. They also place greater demands on the dynamic range required of digital receivers and their analog-to- digital convertors.
Present receivers address the problem of interference by using frequency-notch filters and multiple stages of frequency conversion to remove known interference. Also, multiple stages of automatic gain control (AGC) and limiting are used to prevent saturation of the electronics and to extend their linear range to higher input power levels. However, multiple stages of AGC and/or limiting reduce the sensitivity of the sensor. Actual systems might include more than 75 dB of gain reduction, distributed along the entire receive path, with an accompanying degradation of the noise figure. Prior electronic limiters typically are amplifiers whose gains become clamped once the intensity of the composite input signal reaches or exceeds a certain value. That clamping has no frequency selectivity and applies to all frequency components of the input. Thus, desired frequency components are also adversely affected. AGC amplifiers likewise have no frequency selectivity. What is needed is a limiting system and method for selectively attenuating certain frequency components while not attenuating other frequency components.
Stimulated Brillouin Scattering (SBS) has been used to selectively attenuate the optical carrier of an amplitude-modulated RF lightwave signal, see U.S. Statutory Invention Registration H 1,791 entitled “Stimulated Brillouin Scattering For Fiber-Optic Links” published Mar. 2, 1999 and Electronic Letters, vol. 30, no. 23, pp. 1965–1966 (1994) by Williams and Esner, both of which are hereby incorporated herein by reference.
SBS is a known optical effect. When an optical frequency electromagnetic wave causes vibrations (i.e. an acoustic wave) of the density of an optical medium, an optical grating is produced that causes scattering of the electromagnetic wave traveling in the optical medium. In Brillouin scattering, the wavelength of the scattered electromagnetic wave is shifted with respect to that of the original electromagnetic wave due to the Doppler effect from the motion of the acoustic wave. The frequency shift is a maximum in the backward direction and it reduces to zero in the forward direction, which makes SBS a mainly backward directed process. The incident optical frequency is also known as the pump frequency, which gives the Stokes and anti-Stokes components of the scattered radiation.
In U.S. Statutory Invention Registration H 1,791, the threshold for SBS, which typically depends on the length of the optical fiber and the level of the optical power input to the fiber, is set to attenuate just the optical carrier and not attenuate the modulation sidebands or other frequency components. The effect of this selective attenuation is to enhance the modulation depth (the ratio of the modulation sideband to the carrier). The increased modulation depth can improve the performance of the RF-photonic link. H 1,791 makes use of either a long length of optical fiber or a weakly coupled fiber-optic ring resonator as the medium in which the SBS attenuation occurs. The purpose of the ring resonator is to increase the effective length of the SBS medium so that the optical path-length is much longer than the physical length of the optical filter.
In U.S. Pat. No. 6,178,036 to Yao and in IEEE Photonics Letters, vol. 10, no. 1, pp. 138–140 (1998), SBS is used to selectively amplify a selected RF-lightwave modulation sideband. An optical pump signal is injected in the reverse direction into the SBS medium. The optical pump signal is offset in frequency by the Stokes shift from the desired modulation sideband. Since the frequency of the desired modulation sideband coincides with the frequency of the Brillouin scattering, the sideband is amplified. The purpose of the selective amplification is to selectively amplify the desired modulation sideband and leave the strong carrier un-amplified. This improves the modulation depth. It also produces a single-sideband modulated signal, which may have benefits of reduced distortion from optical fiber dispersion. Amplitude modulation of a carrier can produce two modulation sidebands, which have the same magnitude of frequency offset from the carrier but are offset by positive and negative values, respectively. In a single-sideband modulated signal, one sideband of this pair is substantially stronger than the other sideband.
The prior art discussed above utilizes the well-known SBS effect to improve the modulation depth of a RF-modulated lightwave signal or to reduce the distortion from optical fiber dispersion.
The method and system disclosed herein exploits the narrow-band power limiting action of SBS to suppress strong interfering signals, while minimally affecting the desired low-power and/or wide bandwidth received signal.
The relatively narrow bandwidth of gain for SBS in optical fibers is used to produce a peak-power limiter for undesired RF and RF-lightwave signals. The RF signals are amplitude modulated onto a lightwave carrier to create modulation sidebands. The limiter selectively attenuates those modulation sidebands that are stronger than a threshold level.
Thus, only strong frequency components are limited and the weaker frequency components become enhanced, in comparison. The advantage of the SBS approach to limiting is that it is passive, it self-selects the frequencies attenuated, and it affects only a narrow band at each attenuation notch. The SBS only affects a narrow band at each attenuation notch because the spontaneous bandwidth of the SBS effect in an optical fiber is typically smaller than 100 MHz. The system and method disclosed makes use of the relatively small bandwidth of the SBS effect to distinguish between the different modulation sidebands, which are spaced farther apart than the SBS gain bandwidth. Such frequency selective limiting is not normally achievable with electronic limiters. In addition, the disclosed system and method seeds the Stokes-shifted light into the main SBS medium (in the reverse direction) so that the length of that main SBS medium can be reduced. This seed is preferably generated in a separate ring or strand of fiber from the main SBS medium.
Previously, as disclosed in U.S. Statutory Invention Registration H 1,791, a long length of fiber (generally 25 km or greater) is used to sustain the SBS at common levels of optical power, generally below 10 mW for the frequency components attenuated. It would be desirable to either reduce the amount of optical power required and/or to shorten the length of the SBS medium 207 to thereby improve the system's signal-to-noise ratio. The use of shorter fibers in the SBS medium 207 would have reduced passive losses as compared to longer fibers required by the teaching of the prior art. Further, the use of shorter fibers for the SBS medium also has the advantage of reducing the four-wave mixing of multiple signal frequencies contained in the signal. Such four-wave mixing generated signals can create spurious noise in a long SBS fiber medium.